Doneness indicator for heat-in packaging and method of making same

ABSTRACT

A heat-in package with doneness indicator for heating applications, such as microwave heating of foods or sterilization applications. The doneness indicator comprises indicia printed with thermochromatic ink on the heat-in packaging. The doneness indicator can be sealed from front to back on the heat-in packaging in such a way that the heat transfer to the sealed area is controlled so that the contents contained within the heat-in packaging has adequate time to heat before a color change or other visual transformation is observed in the doneness indicator. Furthermore, the sealed area containing the thermochromatic ink is not in direct contact with the contents within the packaging. The ink changes due to heat conduction through the film. A venting system can further be incorporated to prevent excessive pressure build-up within the packaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. 119(e)(1) of a provisional patent application Ser. No. 61/176,397, filed May 7, 2009, which is incorporated herein by reference in its entity.

FIELD OF THE INVENTION

The present invention is generally directed to doneness indicators. More particularly, the present invention is directed to a heat-in package, such as a flexible plastic bag, having a doneness indicator utilizing thermochromatic ink.

BACKGROUND OF THE INVENTION

Consumers often prefer to cook food in a microwave oven rather than conventional ovens because of the reduced cooking time required to heat foods in a microwave oven. As a result, a wide variety of food items have been designed for heating in a microwave oven. Popular examples of these items include pasta dishes, vegetables and vegetable casseroles, pizza items, and the like.

Heat-in packaged food products are becoming more popular because of their ease of preparation and clean-up. Heat-in packaged food products are essentially foods heated in the package in which they are distributed to the consumer or other purchaser. Heat-in packaged food products can be heated or warmed with the application of heat via stovetop or microwave. With some heat-in packaged foods, the application of heat produces steam within the packaging. The packaging creates an adequate seal such that the steam is maintained within the bag for at least the time necessary to adequately heat the food product, i.e. vegetables, to its desired doneness for consumption. Such doneness can be determined, for example, by temperature, flavor, visual, texture, and/or safety characteristics.

Many food products, such as, for example, vegetables, popcorn, pastas, sauces, and the like, are often available as a heat-in packaged food product. However, it can be difficult to cook these types of food products consistently in the microwave due to variations in cook times, in part due to different microwave wattages. Furthermore, one cannot adequately monitor the doneness of the food product contained within the package, because opening of the packaging can release the steam or heat needed to heat the food product.

Doneness indicators are commonly used in the food industry to provide the consumer with a visual cue for certain “doneness” parameters such as, for example, readiness for consumption or use, general hot or cold indication, specific temperature readouts, spoilage, and other such indications. A visual indicator can be used in which indicia, such as printed indicia, changes in color, appearance, transparency, shape, and combinations thereof. One specific application is the use of thermochromatic ink to indicate a change in temperature of a product or its packaging. Thermochromatic ink is defined as ink that changes color or transparency in response to changes in temperature. Thermochromatic inks can be reversible in which the colors change back and forth corresponding to temperature changes, or irreversible in which the color change is permanent.

Examples of food products that currently use thermochromatic ink indicators include Hungry Jack® microwavable syrup, Coors Light Cold Activated Bottle, Go-Gurt®, and a variety of other products. Mountain graphics on the Coors Light Cold Activated Bottle turn from white to blue when the bottle and its contents are cooled to a desired imbibing temperature. Yoplait's® promotional cup and Yoplait's® Go-Gurt® product provide promotional play or entertainment value for the consumer by revealing a message or image as the temperature sensitive ink reversibly changes color when the product temperature changes from a refrigerator operating temperature in a range of from about 32 degrees to about 42 degrees Fahrenheit to an operable temperature of about 46 degrees Fahrenheit to about 61 degrees Fahrenheit. The Hungry Jack® microwavable syrup bottle is a rigid plastic bottle with thermochromatic ink indicator printed or applied to an exterior of the bottle such that the label or indicator area is in close proximity to the syrup food product, separated only by the plastic wall and an optional film barrier. The indicator changes color when the syrup is warmed to a temperature such that the heat from the warmed syrup transfers through the bottle wall to fire the ink.

However, the doneness indicators utilizing thermochromatic inks as described above are not suitable for many types of heat-in packaged products because they are in close proximity to the actual food product and fire in response to the surface temperature of the food product, and therefore do not necessarily correlate to a required “doneness” of a food product, such as, for example, desirable or acceptable sensory attributes. For example, many types of vegetables need extended time at elevated temperatures to reach the desired doneness for acceptable sensory attributes, flavor, visual, texture, as well as safety characteristics. Furthermore, “doneness” may require that the food product reach a certain “lethality” benchmark to ensure safe consumption, such as an internal temperature that the food product must reach, such as, for example, 160 degrees Fahrenheit, or a certain internal temperature that the food product must maintain for an extended period of time. Doneness indicators commonly used in the food industry today placed on a heat-in package would activate upon the heating of the packaging or the air in direct contact with the indicator, which is not necessarily correlated to the desired doneness or lethality benchmark of the food product.

There remains a need for a more sophisticated doneness indicator for heat-in packaging such that the indicator adequately relays to the consumer that the packaged product is ready for use, such as a food product that is ready for consumption, and that the product meets a doneness benchmark correlated not only by a temperature of the product, but also sensory attributes and/or lethality or safety standards.

SUMMARY OF THE INVENTION

A heat-in packaging system with doneness indicator according to embodiments of the present invention overcomes the deficiencies described above. The doneness indicator can be partially or completely isolated from the contents of the package by sealing. Furthermore, the sealed area containing the thermochromatic ink is not in direct contact with or in close proximity to the contents contained within the packaging. The thermochromatic ink of the indicator changes due to heat conduction through the film into the indicator area. Therefore, the combination of the seal and the placement of the doneness indicator allows the heat transfer to the sealed area containing the thermochromatic ink of the indicator to be controlled so that the product contained within the heat-in packaging has adequate time to heat before a color change is observed in the doneness indicator.

In some embodiments of the invention, a heat-in packaging system can be used in sterilization applications, such as autoclaving, in the medical, dental, and/or pharmaceutical industries. A tool or pharmaceutical composition can be hermetically sealed within the package. The activation or “firing” of the fully or partially isolated doneness indicator corresponds to the adequate time and/or temperature required to adequately sterilize the contents of the package.

In other embodiments of the invention described in more detail below, a heat-in packaging system is used in the food industry for microwave heating or cooking of food products. The activation or “firing” of the fully or partially isolated doneness indicator corresponds to the adequate time and/or temperature required to achieve desired “doneness” of the food product contained within the package. Such doneness can be measured by subjective sensory attributes, such as desirable flavor, texture, and/or visual characteristics, and can further be measured by objective lethality or safety standards.

In one embodiment of the invention, a heat-in packaging system includes a venting system. The heating of the contents of the package increases the pressure within the hermetically sealed package. A venting system is included to prevent the packaging from exploding before the heating process is completed. One such venting system can comprise a controlled-release venting system.

The above summary of the invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a heat-in packaging with doneness indicator according to an embodiment of the invention.

FIG. 2 is a rear view of the heat-in packaging with doneness indicator of FIG. 1.

FIG. 3 is a cross-sectional view of a laminated film assembly used in forming a heat-in package with doneness indicator according to an embodiment of the invention.

FIG. 4 is a perspective view of a bagger for forming a heat-in package with doneness indicator according to an embodiment of the invention.

FIG. 5 is a control test package with doneness indicator that has not been heated.

FIG. 6 is a test package with doneness indicator that has been heated sufficiently to activate the doneness indicator.

FIG. 7 is a test package with doneness indicator that has been heated but has not activated the doneness indicator.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 1 and 2, a heat-in package system with doneness indicator 100 generally comprises a flexible film package 102 having a first major surface 104 and a second major surface 106. Package 102 is sealed with edge seals 108 on at least one edge to secure first major surface 104 to second major surface 106, forming a bag or pouch for containing an item or composition, such as a food product, medical or dental instrument, pharmaceutical composition, and the like. Package 102 can then be completely, hermetically sealed on all edges to contain and secure the contents within package 102. In one embodiment of the invention, as shown in FIGS. 1 and 2, package 102 is sealed on each edge with seals 108, with a single back seam 110 on second major surface 106.

For exemplary purposes only, a heat-in package system with doneness indicator 100 is described below for use in the food industry for microwave heating or cooking of food products. However, heat-in package system 100 is not limited to this application.

Package 102 can comprise any suitable shape, such as, for example, square, circular, rectangular, triangular, and the like, to secure a food product within. Package 102 can comprise a flexible plastic film, such as, for example, a polyester, polypropylene (PP), polyethylene (PE), PP/PE copolymers, and any other suitable polymer, copolymer, or combinations thereof for forming a flexible film package or bag. In one embodiment of the invention, package 102 comprises a flexible film that is microwavable and/or heat sealable. In an alternate embodiment of the invention, package 102 can comprises a rigid plastic material, such as high density polyethylene (HDPE), polystyrene, polypropylene, and combinations thereof, to form a rigid container, such as a tray with a cover. The rigid plastic package can be formed from injection molding or other similar processes. For exemplary purposes only, the description herein is in reference to a flexible plastic package; however the invention is not limited to such.

Referring to FIG. 3, package 102 can also comprise laminated film assembly 112 comprising an outer layer 114 laminated, with adhesive for example, to an inner layer 116. In the case of laminated film assembly 112, outer film 114 is not in contact with the food product, and only the inner surface of inner film 116 is in contact with the food product. Outer film 114 and inner film 116 can be formed from any combination of the above mentioned flexible plastic film. In one embodiment of the invention, inner film 116 comprises a PP/PE copolymer having a thickness in a range from about 1 to about 4 mils, and more specifically about 3.0 mils, and outer film 114 comprises a thin layer of a polyester film, such as PET or PP having a thickness in a range from about 48 gauge to about 100 gauge or more.

Referring back to FIG. 1, package 102 includes a doneness indicator 118 positioned in a corner, or top, head section of package 102, such that indicator 118 is positioned above the contents sealed within package 102. For example, with respect to a heat-in packaging system 100 for a food product, the food product can occupy about sixty percent or less of an inner volume of package 102. When package 102 is placed within a microwave, package 102 can take on a stand-up pouch configuration in which a base is formed at the bottom of package 102 and the food contents settle towards the base such that a head space is formed toward the upper half of package 102. Indicator 118 is positioned within this head space of package 102 such that it is not in close proximity to the food product.

In an alternative embodiment of the invention, package 102 is placed within a microwave on its side such that the contents sealed within package 102 is in contact with close proximity to, although not in direct contact with doneness indicator 118. As package 102 pressurizes, the film of package 102 will lift away from the contents or food product, thereby forming a headspace, similar to the headspace formed when the package is in the stand-up pouch configuration described above.

Doneness indicator 118 generally can comprise any suitable printed indicia, such as text, graphics, logos, and combinations thereof. In one embodiment of the invention, doneness indicator 118 is formed from a thermochromatic ink that changes color or transparency as the temperature of the surrounding film, material, or air changes. An example of a suitable high-temperature thermochromatic ink is ThermaSOLV, a reversible ink available from Sun Chemical. Typically, such thermochromatic inks activate at temperatures from about 158 degrees Fahrenheit (70 degrees Celsius) to about 194 degrees Fahrenheit (90 degrees Celsius). Doneness indicator 118 can be printed directly on either the exterior of package 102 or an interior of package 102 by any suitable printing methodologies, such as, for example, lithography, flexography, ink jet, or combinations thereof. Suitable inks can be water-based inks, water-based encapsulated inks, such as wax melts, in which the encapsulated inks are suspended in water, or solventless inks. In one embodiment of the invention, doneness indicator 118 is press-applied to package 102. In an alternative embodiment, doneness indicator 118 comprises a label laminated to package 102 by either a temperature sensitive or mechanical, i.e. pressure sensitive, attachment means.

Referring again to FIG. 3, in one embodiment of the invention, when package 102 comprises laminated film assembly 112, doneness indicator 118 can be applied between the outer and inner films 114 and 116, i.e. either on the inner surface of outer film 114, or the outer surface of inner film 116. As depicted, doneness indicator 118 is applied to first major surface 120 of outer film 114 either by printing directly on surface 120 or laminating it thereto. In an alternative embodiment not shown, doneness indicator 118 is applied to a second opposing surface or exterior surface of package 120.

Referring back to FIG. 3, adhesive 122, such as adhesive in the form of either a continuous layer or discontinuous layer, is applied over doneness indicator 118 such that doneness indicator 118 is sandwiched between outer film 114 and inner film 116. In this embodiment, doneness indicator 118 is protected from changes to the ambient air temperature, while it at least partially isolated from the food product contained within package 102.

In an alternative embodiment of the invention not shown, an adhesive is applied such that there is no adhesive in contact with or encapsulating the thermochromatic ink of doneness indicator 118. This is to ensure that the depth of color of the thermochromatic ink is fully utilized such that doneness indicator 118 is easily read.

As depicted in FIGS. 1 and 2, package 102 further comprises a perimeter seal 124 around doneness indicator 118 extending from front to back of package 102 to form sealed area 126, resembling a pocket-type area. Perimeter seal 124 can be formed by heat sealing, lower temperature sealing methods such as ultrasonic welding, or other suitable sealing techniques. Lower temperature sealing methods are defined as sealing methods that do not require an application of heat to create the seal, but rather generate heat in response to an application of energy in an alternative form to heat. For example, ultrasonic welding causes local melting of the plastic due to absorption of vibration energy. Lower temperature sealing methods, particularly ultrasonic welding, is preferred to heat sealing, because heat sealing if not done within tightly defined parameters, can prematurely trigger the change in the thermochromatic ink. Further, ultrasonic welding allows for precise localization of the melting process. Ultrasonic welding is well known in the food industry as it is used for membrane packaging of products such as, for example, peanut butter and salsa.

Perimeter seal 124 can be any suitable shape, such as, for example, a circle such that perimeter seal 124 is equidistance from doneness indicator 118 at all places along perimeter seal 124. In another embodiment, sides or edges of perimeter seal 124 are not equidistant from doneness indicator 118 to accommodate manufacturing registration variability while achieving adequate firing of doneness indicator 118.

Perimeter seal 124 can be formed at a distance “x” from a side edge seal 108 of package 102 and a distance “y” from a bottom edge of package 102. Perimeter seal 124 can also be formed at a distance “l” around doneness indicator 118. The dimensions x, y, and/or l can be varied depending on the food product and the time needed for heating the food product. The “y” dimension should be a sufficient length such that doneness indicator 118 is positioned in the head space of package 102 during heating, and not in close proximity to the contents within package 102.

In an alternative embodiment of the invention, package 102 comprises multiple perimeter seals around doneness indicator 118, such as in the form of concentric circles to further slow heating of the film around doneness indicator 118. Concentric circles are preferred over a wider seal area because a wider seal area can cause weakening of the package due to the heat generated by the ultrasonic welding in creating the wider seal.

In yet another embodiment, perimeter seal 124 is comprises a semi-circle such that doneness indicator is only partially sealed, creating a pouch with an opening facing the top of package 102, similar to a semi-circular pocket. In this embodiment, the semi-circle delivery will speed up the transformation of the ink used due to the combination of thermal transfer and direct contact with the generated steam at some point in the cooking process. The bag inflates and at some point the pressure forces steam into the button or interior through the opening in the semi-circle. The amount of time needed to detect a change in the thermochromatic ink depends on the size of the semi-circle opening, and/or the location of the semi-circle relative to the body of the bag.

Sealed area 126 acts as a heat transfer control by controlling the firing or activation of doneness indicator 118, which is correlated to the doneness of the product contained within package 102. Whether package 102 is placed within a microwave in the stand-up configuration or a lying down configuration described supra, the heat generated in the headspace is transferred across seal 124 to fire the thermochromatic ink of doneness indicator 118. Sealed area 126 thereby allows for additional time for heating or cooking of the food contents within package 102 before a change is observed in doneness indicator 118. For example, a fully sealed or isolated doneness indicator can take an additional thirty to 240 seconds for adequate heat to transfer to indicator 118, compared to a non-sealed or non-isolated indicator. The time for adequate heat transfer can depend on a number of variables including microwave variations in wattage, age, and/or brand, food product variations such as density and/or sauced or non-sauced, fill weight variation, and size and/or diameter of the seal. Rather than reacting to the surface temperature of the package similar to the doneness indicators mentioned in the Background section supra, sealed area 126 allows for an indirect temperature reading correlating to a doneness of the food product, acting as a fuse to doneness indicator 118. Further, because of the chemistry of the inks, microwaves themselves do not affect doneness indicator 118.

Packaging system 100 can also include a venting system (not shown). A venting system allows the package to increase in pressure as a result of heating of the hermetically sealed package, while allowing it to vent to prevent over pressurizing the package which can result in an explosion of the bag. A suitable venting system exhibits a controlled venting cycle such that the venting system allows the package to build pressure upon heating so that the package acts as a pressure cooker, while depressurizing the package to avoid explosion. A suitable venting system can comprise precut holes in the package, instructions instructing the end-user to make holes in the package or to cut a corner of the package, or a sophisticated venting system such as Bemis® Magic Steam™ Microwavable Systems.

According to one embodiment of the invention, a method of making heat-in packaging 100 with doneness indicator 118 can utilize a standard bagger known to one of skill in the art. A suitable bagger 130 can comprise a vertical form fill seal (VFFS) machine, such as those available from the Bosch Packaging Technology, a division of Robert Bosch GmbH, or a horizontal form fill seal (HFFS) machine, such as the Hayssen RT2000 model available from HayssenSandiacre of Duncan, S.C. The operation and configuration of each are incorporated herein by reference.

As depicted in FIG. 4, an exemplary belt-driven VFFS bagger 130 can generally comprise a funnel 134 coupled to a first end 136 of an elongated tubular portion 138. A web assembly 140 carries to and moves a flexible film 142 around and longitudinally along elongated tubular portion 138. Flexible film or film laminate can be preprinted with graphics, text, doneness indicator 118, and combinations thereof. A first sealing assembly 144, such as a heat sealer, ultrasonic sealer, pressure sealer, or the like, is used to form a first seam or back seam 110 of package 102. A second sealing assembly 146 which can be the same or different configuration of first sealing assembly 144, is used to form a second edge seam 108 a. A pre-determined portion of food product is introduced into funnel 134 such that the semi-sealed package is filled to a pre-determined level of food product. A sealing assembly 132, such as an ultrasonic welder or its equivalent, is used to form seal 124 and sealed area 126 at a predetermined position on package 102 either before or after the food product is introduced into the semi-sealed package. Ultrasonic welder 132 can generally comprises an anvil, and a sonotrode or horn, connected to a transducer for emitting acoustic vibrations. The film is positioned between the anvil and the sonotrode.

The filled semi-sealed package is then moved off of a second end 148 of elongated tubular portion 138, such that it is supported by the flexible film web only. Second sealing assembly 146 then simultaneously forms a third edge seam 108 b of package 102 and the edge seam 108 a of another package 102 being formed on elongated tubular portion 138.

FIGS. 5-7 are testing bags from a design of experiment in which a number of different variables were explored. Such variables can include, for example, distance “l” from the perimeter seal 124 to doneness indicator 118 (thereby varying sealed area 126), distance “x” from a side edge seal 108 of package 102, distance “y” from the bottom edge of package 102, the distance from a filling point of the package, heating times and temperatures, ink type, coating weights of ink, shape of perimeter seal 124, and other such variables.

The control packages of FIGS. 5-7 are configured such that a pattern develops upon sufficient heating of the contents of the package. The package illustrated in FIG. 5 represents the control package in which no heating was applied, such that no pattern is observed on either the outer surface of the package, or within the sealed area of the package. The package illustrated in FIG. 6 represents a package that was heated such that the pattern was observed on both the outer surface of the package, as well as within the sealed area, indicating that the temperature of the sealed area was sufficient to activate the doneness indicator. The package illustrated in FIG. 7 represent a package that was heated such that some pattern was observed on the outer surface of the package, but not within the sealed area indicating that the temperature of the sealed area was insufficient to activate the doneness indicator.

In one embodiment of the invention, the temperature of the food product was measured at various times after heating began. Each package was filled with twelve ounces of a corn and butter product. The following is a table including the temperature of the food product and the observed pattern of the package. The surface area of the sealed area around the doneness indicator stayed constant. The temperature of the product was measured using standard thermocouples.

Test package Time heated Temperature Indicator fired? 1 (Control) None Product temp No 2 3 minutes 109° F. No; pattern observed on some of outer surface of package only 3 3.5 minutes   160° F. Yes; pattern observed on outer surface of package, and lightly within sealed area 4 4 minutes 198° F. Yes; pattern observed on outer surface of package, and within sealed area 5 5 minutes 215° F. Yes; pattern observed on outer surface of package, and within sealed area

The invention therefore addresses and resolves many of the deficiencies and drawbacks previously identified. The invention may be embodied in other specific forms without departing from the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive. 

1. A heat-in package for heating a food product, the heat-in package comprising: a food product; a flexible plastic film sealed to form a package, wherein the package is adapted to completely encapsulate and retain the food product within the package during heating of the food product; and a doneness indicator adapted to undergo a visual transformation in response to a temperature change, the doneness indicator being at least partially isolated from the food product by an indicator seal, wherein the indicator seal inhibits heat transfer to the doneness indicator so as to delay the visual transformation such that the food product is maintained at an elevated temperature for a period of time to sufficiently heat the food product before the visual transformation is observed through the flexible plastic film.
 2. The heat-in package of claim 1, wherein the doneness indicator is formed from a thermochromatic ink.
 3. The heat-in package of claim 1, wherein the doneness indicator is sandwiched between opposing inner surfaces of the flexible plastic film.
 4. The heat-in package of claim 1, wherein the flexible plastic film comprises a multilayer film.
 5. The heat-in package of claim 4, wherein the multilayer film is formed of two layers, and wherein the doneness indicator is sandwiched between an inner surface of the first layer of the multilayer film, and an opposing inner surface of the second layer of the multilayer film.
 6. The heat-in package of claim 1, wherein at least a portion of the flexible film is transparent or semi-transparent.
 7. The heat-in package of claim 1, wherein the indicator seal is formed on an outer surface of the package by ultrasonic welding.
 8. The heat-in package of claim 1, wherein the food product is selected from the group comprising vegetable, fruits, meat, popcorn, sauce, and combinations thereof.
 9. The heat-in package of claim 1, wherein the indicator seal comprises at least one circle.
 10. The heat-in package of claim 9, wherein the indicator seal comprises a plurality of circles in concentric arrangement.
 11. A method of making a heat-in package having a doneness indicator for heating a food product, the method comprising: providing a food product; forming a flexible plastic film into a package by sealing, the package adapted to completely encapsulate and retain the food product within the package during heating of the food product; positioning a doneness indicator on the package, wherein the doneness indicator is adapted to undergo a visual transformation in response to a temperature change; and sealing the doneness indicator such that the doneness indicator is sealingly isolated from the food product by an indicator seal, wherein the indicator seal inhibits heat transfer to the doneness indicator so as to delay the visual transformation such that the food product is maintained at an elevated temperature for a period of time to sufficiently heat the food product before the visual transformation is observed through the flexible plastic film.
 12. The method of claim 11, wherein positioning a doneness indicator on the package comprises printing a thermochromatic ink on one of the opposing inner surfaces of the flexible plastic film, or an exterior surface of the flexible plastic film.
 13. The method of claim 11, wherein sealing the doneness indicator comprises forming the indicator seal by ultrasonic welding.
 14. The method of claim 11, wherein the doneness indicator is sealed between opposing inner surfaces of the flexible plastic film.
 15. A heat-in package for heating a food product comprising: a food product; a flexible plastic film sealed to form a package, wherein the package is adapted to completely encapsulate and retain the food product within the package during heating of the food product; and a doneness indicator formed from a thermochromatic ink, wherein the doneness indicator is adapted to undergo a color change in response to a temperature change, the doneness indicator being isolated from the food product by an indicator seal formed by low temperature sealing, wherein the indicator seal inhibits heat transfer to the doneness indicator so as to delay the color change such that the food product is maintained at an elevated temperature for a period of time to sufficiently heat the food product before the color change is observed through the flexible plastic film.
 16. The heat-in package of claim 15, wherein the low temperature sealing is accomplished through ultrasonic welding.
 17. The heat-in package of claim 15, wherein the doneness indicator is sandwiched between opposing inner surfaces of the flexible plastic film.
 18. The heat-in package of claim 15, wherein the flexible plastic film comprises a multilayer film.
 19. The heat-in package of claim 18, wherein the multilayer film is formed of two layers, and wherein the doneness indicator is further sandwiched between an inner surface of the first layer of the multilayer film, and an opposing inner surface of the second layer of the multilayer film.
 20. The heat-in package of claim 15, wherein at least a portion of the flexible film is transparent or semi-transparent.
 21. A method of making a heat-in package having a thermochromatic doneness indicator, the method comprising: providing a food product; heat sealing a flexible plastic film to form a package, the package adapted to completely encapsulate and retain the food product within the package during heating of the food product; positioning a thermochromatic doneness indicator on the package, wherein the doneness indicator is adapted to undergo a color change in response to a temperature change; and sealing the thermochromatic doneness indicator by low temperature sealing, such that the doneness indicator is sealingly isolated from the food product by an indicator seal, wherein the indicator seal inhibits heat transfer to the thermochromatic doneness indicator so as to delay the color change such that the food product is maintained at an elevated temperature for a period of time to sufficiently heat the food product before the color change is observed through the flexible plastic film.
 22. The method of claim 21, wherein positioning a thermochromatic doneness indicator on the package comprises printing a thermochromatic ink on one of the opposing inner surfaces of the flexible plastic film, or on an exterior surface of the flexible plastic film.
 23. The method of claim 21, wherein the thermochromatic doneness indicator is sealed sandwiched between opposing inner surfaces of the flexible plastic film.
 24. The method of claim 21, wherein sealing the thermochromatic doneness indicator comprises forming the indicator seal by ultrasonic welding.
 25. A heat-in package for heating contents within the package, the heat-in package comprising: contents to be heated; a plastic film sealed to form a package, wherein the package is adapted to completely encapsulate and retain the contents within the package during heating of the contents; and a doneness indicator adapted to undergo a visual transformation in response to a temperature change, the doneness indicator being at least partially isolated from the contents of the package by an indicator seal, wherein the indicator seal inhibits heat transfer to the doneness indicator so as to delay the visual transformation such that the contents of the package are maintained at an elevated temperature for a period of time to sufficiently heat the contents before the visual transformation is observed through the plastic film.
 26. The heat-in package of claim 25, wherein the contents is selected from the group comprising food products, medical instruments, dental instruments, pharmaceutical compositions, and combinations thereof.
 27. The heat-in package of claim 25, wherein the doneness indicator is sandwiched between opposing inner surfaces of the plastic film.
 28. The heat-in package of claim 25, wherein the indicator seal is formed on an outer surface of the package by ultrasonic welding.
 29. The heat-in package of claim 25, wherein the indicator seal comprises at least one circle.
 30. The heat-in package of claim 25, wherein the indicator seal comprises a plurality of circles in concentric arrangement. 